Royal Mail’s defined benefit (DB) scheme could be closed to future accrual after its contributions are set to more than double from 2018.
A deterioration in financial market conditions, particularly following the Brexit vote, means the cost of running the pension fund would increase to over £900m.
The scheme is now said to be discussing the plan, which could affect around 90,000 employees, with trade unions.
A spokesperson for Royal Mail said: "We understand how much our people value their pension benefits. We committed to keep the Royal Mail Pension Plan open to future accrual on a career average basis for existing members without further changes, at least until March 2018.
"Early indications from the latest triennial valuation of the plan suggest that the company's contributions to the pension plan each year would have to increase from around £400m to over £900m.
"Such an increase in costs is not sustainable. We are talking to our unions about the future of the plan after March 2018."
The announcement follows a similar proposal from the Post Office, which suggested it would close the Post Office section of the Royal Mail scheme to future accrual by March 2017, with members being transferred to its defined contribution (DC) scheme.
The Post Office is now facing strike action, organised by Unite, and it is likely a decision by Royal Mail could lead to a similar response from unions.
The Post Office and Royal Mail were split off from each other in 2013, when the government made the decision to privatise Royal Mail, while the Post Office remained in public hands.
According to the scheme's latest annual update, it had a £3.4bn surplus as of 27 March 2016 under the IAS 19 accounting measure. Royal Mail also recorded £7.4bn of quoted and unquoted assets across its two DB schemes as of 27 March.